Fit-3 Ligand (FL) is a growth factor for DCs, and induces a type 1 T cell response. We recently reported that FL prevented ovalbumin-induced allergic airway inflammation in mice and suppressed late allergic response (LAR) and airway hyperresponsiveness (AHR). Based on these studies we developed the hypothesis that FL has therapeutic activity for asthma via cellular immunoregulatory mechanism that are allergen non-specific. In Specific Aim 1, we will examine the ability of FL to reverse LAR, AHR and eosinophilia in a mouse model of allergic airway inflammation and augment an antigen-specific, type 1 T cell response to the inciting allergen. We will determine the dose-response for FL therapeutic activity and duration of effect. We will also examine the effect of FL on clinical correlates of asthma including baseline AHR in mice sensitized, but not challenged with the allergen. Further we will examine the levels and isotype of antibodies to the allergen and cytokine levels in serum and lung washings, in addition to non-antigen and antigen-specific type 1 and 2 T cell responses by Elispot assays both systemically (spleen) and regionally (lymph nodes and collagenase digested lungs). In Specific Aim 2, we will determine whether FL therapeutic activity in acute and chronic asthma is due to IL-12 regulation of a type 1 immune response. We will determine the therapeutic activity of FL in wild type and IL-12b knock-out (KO) mice, and also examine the therapeutic activity of FL in animals given neutralizing antibodies to IL-12 during allergic airway inflammation. The immune function studies will focus on both DC and T-cells from draining lymph nodes and lungs to examine the underlying mechanisms of FL-induced regulation of type 2 cytokine responses. In Specific Aim 3, we will determine whether FL therapeutic activity in acute and chronic asthma is due to tolerance induction via expansion of immature DCs and/or CD4+-CD25+ regulatory T cells (Tr cells). Further, using cellular adoptive transfer studies we will identify the ability of DCs and Tr cells from naive and antigen-sensitized mice treated with FL to protect against and also treat antigen-specific asthma. We will also examine the histopathology of asthma and spatial relationship with immune cells and examine the production of type 1 and 2 cytokines. The long-term goal of this study is to better define the potential mechanisms of action of FL to prevent and reverse allergic airway inflammation and associated changes in pulmonary function.